I used some old Direct Connection frame connectors on my 71. They are regtangular, like 1 1/2" X 2.00" or so.
Very easy to put in with no floor cutting. They made a huge difference and are plenty stiff. You don't need 2 X 3" tubing, even 2 X 2" is overkill for a street car. IMO.
Yes you can make your own very easy.

Daytona turbo, please make your picture smaller so we don't have to scroll back and forth.